Even though there has been considerable study of alternative electrochemical systems, the lead-acid battery is still the battery-of-choice for general purpose uses such as starting a vehicle, boat or airplane engine, emergency lighting, electric vehicle motive power, energy buffer storage for solar-electric energy, and field hardware whether industrial or military. These batteries may be periodically charged from a generator.
The conventional lead-acid battery is a multicell structure. Each cell contains a plurality of vertical positive and negative plates formed of lead-based alloy grids containing layers of electrochemically active pastes. The paste on the positive plate when charged contains lead dioxide which is the positive active material. The negative plates contain a negative active material such as sponge lead. This type of battery has been widely used in the automotive industry for many years; there is substantial experience and tooling in place for manufacturing this type of battery; and, the battery and its components are based on readily available materials, are inexpensive to manufacture, and are widely accepted by consumers.
During discharge, however, the lead dioxide in the positive plate (which is a fairly good conductor) is converted to lead sulfate, which is an insulator. The lead sulfate can form an impervious layer encapsulating the lead dioxide particles, which, in turn, limits the utilization to less than 50% of capacity, typically around 30%.
The power output is significantly influenced by the state-of-discharge of the battery, since the lead sulfate provides a circuit resistance whenever the battery is under load. Furthermore, the lead sulfate can grow into large, hard, angular crystals, disrupting the layer of paste on the grid, resulting in flaking and shedding of active material from the grid. Power consumption during charge is also increased due to the presence of the lead sulfate insulator. The lead sulfate crystals on the negative electrode can grow to a large, hard condition and, due to their insulating characteristics, are difficult to reduce to lead. Even when very thin pastes are utilized, the coating of insulating lead sulfate interferes with power output. Thus, power capability is greatly influenced by the state-of-charge of the battery.
An apparent solution to this problem would be the addition of a conductive filler to the paste. The filler should be thermodynamically stable to the electrochemical environment of the cell, both with respect to oxidation and reduction at the potential experienced during charge and discharge of the cell, and to attack by the acid.
Increasing the conductivity of the positive paste by adding a conductive filler such as graphite has been attempted. Graphite has been used successfully as a conductive filler in other electrochemical cells such as, for example, in the manganese dioxide positive active paste of the common carbon-zinc cell and mixed with the sulfur in sodium-sulfur cells.
Even though graphite is usually a fairly inert material, however, it is oxidized to acetic acid in the aggressive electrochemical environment of the lead-acid cell. The acetate ions combine with the lead ion to form lead acetate, a weak salt which is readily soluble in the sulfuric acid electrolyte. Corrosion to the positive grids is the result, especially those parts of the grid wires uncovered by lead oxide. Highly conductive metals such as copper or silver are not capable of withstanding the high potential and strong acid environment present at the positive plate of a lead-acid battery. A few electrochemically-inert metals such as platinum are reasonably stable; but, the scarcity and high cost of such metals prevent their use in high volume commercial applications such as the lead-acid battery. Platinum would be a poor choice even if it could be afforded because of its low gassing-over potentials.